Current trends in the relevant global climate conditions lack any dominating feature likely to cause either a highly active or inactive Atlantic hurricane season this summer. Rather, information obtained through March 1998 indicates that the upcoming 1998 Atlantic hurricane season is likely to experience about average hurricane activity. We project that total season activity will include 10 named storms (average is 9.3), 50 named storm days (average is 47), 6 hurricanes (average is 5.8), 20 hurricane days (average is 24), 2 intense (category 3-4-5) hurricanes (average is 2.3), 4 intense hurricane days (average is 4.7) and a hurricane destruction potential (HDP) of 65 (average is 71). Whereas net 1998 tropical cyclone activity is expected to be about 95 percent of the long term average, conditions this year should be distinctively more active than 1997 but less active than the very busy hurricane seasons of 1995 and 1996. This early April prediction indicates a greater possibility of a slightly more active season than did our early December (1997) forecast of 1998 hurricane activity. An important element entering this April forecast update is more evidence that the very strong 1997-1998 El Niño will be largely dissipated by the start of the (climatologically) active part of the hurricane season (i.e., mid-August). Additional later forecasts updates for 1998 will be issued on 5 June and 6 August 1998. A post-season review and critique of this forecast will be made in late November 1998. These forecasts, as well as those for past seasons, are also available on the World Wide Web at the access location given on the cover page.

1. Introduction

Surprisingly strong long range predictive signals exist for seasonal tropical cyclone activity in the Atlantic basin. Our ongoing research indicates that a sizeable portion of the year-to-year variability of nine indices of Atlantic tropical cyclone activity can be forecast with useful skill as early as late November of the prior year. Each year this late fall forecast is then updated in early April, early June and early August. This paper presents the early April update of our Atlantic 1998 forecast which contain meteorological data available through the end of March 1998.

Our seasonal hurricane forecasts are based on the premise that the behavior of the atmosphere during the coming year will, in general, follow that of similar years of the past. In other words, we assume that those global environmental conditions which preceded active (or inactive) hurricane seasons in the past will be similarly related to future active (or inactive) hurricane seasons. Allowing that seasonal variations of the global atmosphere and ocean tend to occur as a coherent, structured process, past observations provide insight as to how the atmosphere-ocean-land system will likely operate in future months and seasons. As we study new data and ideas, our forecast methodology continues to evolve while our overall forecast skill improves with time.

Forecasts are developed using 48 years (1950-1997) of historical data. We examine this historical data in order to develop the best possible forecast equations from a variety of global wind, temperature, pressure, and rainfall features. Figures 1 and 2 show the various factors which are used either for our statistical models or provide additional considerations for determining our final ``adjusted" forecast.

2. Prediction Methodology

We forecast nine measures of seasonal Atlantic basin tropical cyclone activity including the following seasonal numbers of Named Storms (NS), Named Storm Days (NSD), Hurricanes (H), Hurricane Days (HD), Intense Hurricanes (IH), Intense Hurricane Days (IHD), the Hurricane Destruction Potential (HDP), Net Tropical cyclone Activity (NTC), and the Maximum Potential Destruction (MPD). (Definitions for these indices are given on page 2). For each of these measures, we choose the best three to six predictors (i.e., those resulting in optimum prediction skill) from a group of 13 possible forecast parameters known to be related to tropical cyclone activity. The current set of potential predictors used to develop our early April forecast is shown in Table 1. The specific values of these parameters used in this year's April forecast are shown in the right hand column.

We make a number of statistical forecasts which are summarized in Table 2. Column 1 of Table 2 represents our best statistical forecast where, so as to minimize the skill degradation of these equations when making independent forecasts via statistical ``overfitting", we include the least number of predictors for the highest amount of hindcast variance. We stop adding predictors when the hindcast improvement of the next best predictor adds less than a 0.025 improvement to the total variance explained.

We have also studied a scheme which uses various fixed number of predictors. This procedure investigates how hindcast variance (not necessarily true skill) increases as a fixed number of predictors are increased from 4 to 6. Although independent forecast skill (i.e., ``true skill") typically degrades in approximate proportion to the increased number of predictors, it is of interest to determine the degree of hindcast improvement which occurs with added predictors. Individual year forecast skill degradation from application of hindcast statistics can never be accurately specified. Consequently, as the latter are purely random effects, the hazards of overfitting become obvious.

Additional forecast parameters representing conditions in the Atlantic and Pacific Ocean basins and in the Asia-Australia regions (Figs. 1 and 2) are also consulted for further qualitative interpolation and possible influence on our final ``adjusted" forecast.

Table 2 lists hindcast prediction skills for our various statistical models including the variable number of predictor schemes along with fixed sets of 4 and 6 predictors. Probability dictates that, on average, a net degradation of this hindcast skill of between 5-15 percent of total in variance will likely occur. The amount of degradation (if any) for an individual year forecast is a random process. In some years when conditions include strong trends that are similar to past years, forecasts will do quite well while in other years, a given forecast can perform quite poorly. This is because our 48-year (1950-1997) base of predictors likely does not explain the full range of independent possibilities. Our 1997 forecast is a good example. No year in our 1950 through 1996 developmental data sets had ever experienced an El Niño event anywhere nearly as intense (by a factor of 2) as the 1997-98 El Niño - the most intense event ever measured.

3. Early April Forecast

Table 3 lists our April statistical forecast prediction for the 1998 hurricane season along with what we consider our current best qualitatively adjusted forecast. Columns 1-3 lists all of our statistical forecasts, column 4 contains our best qualitative adjusted ``final" forecasts, and column 5 provides the climatological mean for each parameter for 1950-1990. We have made a small upward adjustment of our early December forecast. An average hurricane season is expected.

We consider the variable predictor scheme to be our best forecast. Table 4 shows the statistical spread of our predictors as well as our hindcast skill and expected likely skill with independent data.

Discussion

Forecast signals for 1998 contain a mix of positive and negative influences. Of the 13 potential predictors listed in Table 1, six (those associated with the QBO and last year's West African rainfall) indicate below average activity, whereas the other seven factors indicate above average activity for 1998. Additional climate factors favorable for 1998 hurricane activity in the Atlantic basin which are not believed to be fully reflected in our objective April predictions include:

1. A large weakening of the Atlantic subtropical ridge or Azores high of -1.37 SD in October-November 1997 and -0.91 March 1998. The weaker than normal Atlantic wind gyre resulting from these pressure decreases acts to reduce the amount of ocean upwelling along Western Africa and South America and leads in warmer waters and lower sea level pressure in the tropical Atlantic this coming summer.
2. Projected equatorial Atlantic SST anomalies in the Gulf of Guinea area south of the West Africa bulge are forecast (IRI, 1998) to become cold while the tropical Atlantic west of Africa (i.e., off the coast of Mauritania and Senegal) is presently warm and is projected to remain warm. If these SSTA patterns do occur (as we expect them to), this should be an enhancing influence for both West African rainfall and associated intense hurricane activity this season. We now expect rainfall conditions for the Western Sahel region this summer to be somewhat above average and higher than those anticipated in our early December 1997 forecast (Landsea et al. 1997).
3. Broad scale North Atlantic SSTA patterns are warm for the fourth consecutive year, implying a multidecadal shift in North Atlantic SSTs has occurred which we believe is related to changes in the Atlantic Ocean thermohaline (''conveyor'') circulation. Persistence of warm SST (anomaly) conditions over the North Atlantic west of the UK during 1997 and for the January-March 1998 period indicates a persistence of a stronger than normal Atlantic Ocean thermohaline circulation. Such ocean circulation changes are typically associated with increased intense or major hurricane activity.
4. More evidence (and hence, greater confidence) that the strongest El Niño on record will be largely dissipated by August 1998. Anomalously cold Pacific equatorial sub-surface ocean water is presently moving upward and eastward. Below average (cold) water should begin upwelling along the eastern Pacific equator in the next 2-3 months. East Pacific subtropical highs in both the Northern and Southern Hemisphere are now becoming stronger and trade winds should soon be established. Both of these influences should lead to a speedy termination of the present El Niño by this summer.
5. Middle latitude Atlantic wind patterns during the winter of 1998 have been more typical of the blocking conditions which were more prevalent during the late 1940s through the late 1960s, a time when the far North Atlantic also had similar warm SSTA conditions and Atlantic intense hurricane activity was more prevalent.
6. Lower than normal SLPA are typically related to increased hurricane activity (Shapiro 1982, Gray 1984, Knaff 1997). An early April 1998 forecast by Knaff (1998) calls for negative SLPA in the Caribbean and western tropical Atlantic for this coming summer. Table 5 provides details of these SLPA forecasts which are based on anomaly information concerning the March Atlantic subtropical ridge, January through March SSTs in the North Atlantic (50-60^°N, 10-50^°W), and January through March Ni~no 3.4 (5^°N-5^°S, 120^°W-170^°W) SST anomalies. Using these factors in separate regression equations leads Knaff to a forecast reduced Caribbean-western tropical Atlantic SLPA for the months of June-July, August-September, June-August and June through September, respectively. Lower pressures should enhance hurricane activity.

In summary, data through the end of March indicate that 1998 will experience hurricane activity above that to be expected for an easterly stratospheric QBO year and in a year when a strong El Niño is still present near the start of the season.

4. Current El Niño Influences on the 1998 Hurricane Season

A difficult aspect of our 1998 forecast is predicting the rate of dissipation of the current El Niño and determining if there are going to be an El Niño induced residual influences on this year's hurricane season. This assessment involves prediction of both the El Niño dissipation rate through summer 1998, as well as gauging delayed El Niño induced negative residual influences. Hurricane seasons in years of fading El Niños can be either active or inactive. For instance, the hurricane seasons of 1919, 1931, 1970, 1973, 1983, and 1992 (following El Niño years of 1918, 1930, 1969, 1972, 1982, and 1991) were quite inactive while those years of 1906, 1915, 1926, 1958, 1966, and 1995 (following the El Niño years of 1905, 1914, 1925, 1957, 1965, and 1994) were quite active. Table 6 lists these years.

We presently believe that the 1998 hurricane season will be more typical of the active hurricane years associated with fading El Niños. Atlantic basin ocean and atmospheric circulation conditions as manifested by the Atlantic subtropical ridge pressure, Atlantic SSTA, Atlantic SLPA, and Western Sahel rainfall are different for these two classes of ENSO weakening years; 1998 Atlantic basin weather conditions being more typical of an active period.

Knaff and Landsea (1997) have recently developed a new extended-range ENSO prediction scheme based on an optimum combination of climatology and persistence. Their SST forecast of June-July-August and September-October-November 1998 for Nino 3.4 SST anomaly indicates values are -0.03^°C and -0.06^°C, respectively. Hence, we believe that the current ENSO will cool rapidly over the next 3-4 months and cool eastern Pacific SSTA conditions will be in place by late summer and early fall. Such a rapid winter to summer reversal is typical of very strong El Niños. Figure 3 indicates how we project the current El Niño to fade out.

Table 8 lists contrasting information for Atlantic basin circulation features associated with analog years which had retreating El Niños versus those retreating El Niño years when Atlantic basin conditions were different than 1998, the ``non-analog" years.

Given the hurricane enhancing warm Atlantic SST and low Azores subtropical ridge conditions, we expect that the negative influences of the likely easterly QBO to be more than canceled out and that a near average 1998 hurricane season will ensue.

5. Likely Cause of Unusually Strong 1997-1998 El Niño

Our 1997 hurricane underestimation was due to our inability to anticipate the coming strength of the strongest El Niño on record. The West Pacific equatorial warm water pool was stronger than we realized. Figure 4 shows that the weak El Niño warming events of 1991 through 1994 probably did not significantly deplete the warm pool. Since the last major El Niño of 1986-87, the warm water pool was very large. This ``poised for release" warm water pool was then set off by a number of 40-50 day oscillation westerly wind events and from the development of a number of long lasting tropical cyclones which also caused equatorial wind to also be from the west. This was a very unusual combination of events.

7. Theory Behind Forecasts and Cautionary Note

It is important that the reader appreciate that these seasonal forecasts are based on statistical schemes which, owing to their intrinsically probabilistic nature, will fail in some years. Moreover, these forecasts do not specifically predict where within the Atlantic basin storms will strike. Regardless of whether 1998 should prove to be an above or a below average hurricane season or not, the probability always exists that one or more hurricanes may strike along the US or Caribbean Basin coastline and do much damage.

8. Anticipated Hurricane Activity for Next Year (1999)

The ensemble of slowly evolving climatic conditions suggest that we will likely have an active hurricane season during the summer of 1999. This speculation is based on expectations that (1) the current El Niño will be fully dissipated and cool SST anomaly conditions will be present during 1999 in the Eastern Equatorial Pacific, (2) relatively warm tropical North Atlantic SST anomaly conditions and a strong thermohaline circulation will persist and (3) stratospheric QBO conditions during 1999 are projected to be from the more favorable westerly direction.

9. Acknowledgements

This research, analysis and forecast have been supported by research grants from the National Science Foundation (NSF) and by the Bermuda RISK Prediction Initiative. The authors are indebted to a number of meteorological experts who have furnished us with the data necessary to make this forecast or who have given us valuable assessments of the current state of global atmospheric and oceanic conditions. We are particularly grateful to John Sheaffer, Todd Kimberlain, Arthur Douglas, Richard Larsen, Ray Zehr and Vern Kousky for very valuable climate discussion and input data. We thank Colin McAdie who has furnished much data necessary to make this forecast and to Gerry Bell, James Angell and Stan Goldenberg for helpful discussions. William Thorson and Richard Taft have provided valuable data development and computer assistance. We wish to thank Tom Ross of NCDC and Wassila Thiao of the African Desk of CPC who provided us with West African and other meteorological information. Douglas LeCompte of USDA provided us with African rainfall summaries. In addition, Barbara Brumit and Amie Hedstrom have provided manuscript and data analysis assistance. We have profited over the years from many indepth discussions with most of the current NHC hurricane forecasters. These include Lixion Avila, Miles Lawrence, Max Mayfield, Richard Pasch, Edward Rappaport and Jack Beven. The first author would further like to acknowledge the encouragement he has received for this type of forecasting research applications from Neil Frank, Robert Sheets, Robert Burpee former directors of the National Hurricane Center (NHC) and from current acting NHC director Jerry Jarrell.

10. Additional Reading

Post-Season Reviews of All Prior Seasonal Forecasts